Idle-channel tones exist in sigma-delta converters. In audio applications, the idle-channel tones can cause unpleasant noise detectable by the human ear. Dithering is the most popular method to reduce the idle-channel tones. One effective dithering method is to add a noise-shaped random series, called dither, in such a way that the dither transfer function is the same as the quantization noise transfer function. A sigma-delta modulator having generalized conventional dither is shown at 10 in FIG. 1. X(n) and y(n) are the input and output, respectively, of the modulator 10. G(z) is the feedforward Z transfer function, and H(z) is the feedback transfer function of the modulator. A pseudorandom series dither d(n) is added to the input of the quantizer.
From literature and simulations, the dithering amplitude must be big enough to remove the idle-channel tones. For example, an 1-bit quantizer, δ/Δ>0.5, where δ is the peak-to-peak range of the dither, and Δ is the quantizer interval. When a fixed-amplitude of dither is applied all the time, the dithering is referred as static dithering. When adding a static dither to a modulator, the noise and distortion characteristics for large input signals are adversely affected. Noise floor of the sigma-delta modulator 10 may increase by several decibels. With static dither, when the input signal is approaching full scale, sigma-delta modulators have reduced dynamic range or dynamic range penalty. To avoid this effect, a dynamic dither that decreases its power when input level increases is preferred.
FIG. 2 shows a prior-art dynamic dither scheme at 20. The input 22 can be an analog signal for an analog-to-digital converter (ADC), or a digital signal for a digital-to-analog converter (DAC). A coarse input power level estimator 24 determines how much of the dither signal d(n) will be adjusted based on the input level of input 22. A quantizer Q, shown at 26, has an output fed back to form the negative-feedback loop. Dither signal d(n) is a random number series. Signal d′(n), which is proportional to dither d(n), can be digital for a DAC or analog for an ADC, and is determined by the output of the coarse input power level estimator 24 and dither d(n). In one example, if the input at 22 is idle or very small, signal d′(n) has a big amplitude, and it will attenuate with the input signal increase. Thus, this dither method 20 is called dynamic dithering. The attenuation factor is normally the function of the input amplitude. For example, (1−|x(n)|α), where α=¼.